1. Field of the Invention
The present invention relates to a spread spectrum communication system and, in particular, to a spread spectrum communication system for compensating an error of a carrier frequency.
The spread spectrum communication system according to the present invention is based on Korean Application No. 16472/1995, which is incorporated herein by reference.
2. Description of the Related Art
In general, upon transmitting data in a spread spectrum communication system, a transmitter sequentially modulates the data, multiplies the modulated signal by a carrier generated from a local oscillator, converts the multiplied signal into a high frequency signal of a desirable band, and transmits the converted signal. Conversely, upon receiving the data in the spread spectrum communication system, a receiver multiplies a received signal by the carrier from a local oscillator, converts the multiplied signal into a signal of a band adequate for demodulation, and demodulates the converted signal by a demodulator.
At this point, in the case that the carrier of the spread spectrum communication system has no exact frequency value, a signal applied to the demodulator causes a frequency offset of as much as .DELTA.f due to errors or offsets between the carrier frequency of the transmitter and the carrier frequency of the receiver. Since such frequency offset negatively affects the operation of the receiver in accurately restoring or receiving the transmitted data, there is a need for minimizing the frequency offset.
An automatic frequency control circuits for measuring the frequency offset and automatically controlling the frequency of the received signal have previously been used in spread spectrum communication systems. In such use, most automatic frequency control circuits were designed to control the frequency of the signal having a phase shift per symbol in a range below 2.pi.. However, if the automatic frequency control circuits were used in a spread spectrum communication system in which the phase shift generated the frequency offset adjacent to 2.pi. or more than 2.pi., a problem existed in that the frequency offset of the received signal could not be removed.
To overcome the above problem, a high precision local oscillator may be used to limit the frequency offset, but a high precision local oscillator is generally expensive.
Therefore, a need exists for a spread spectrum communication system for limiting the effects of frequency offsets regardless of the type of the local oscillator used in the spread spectrum communication system.